[#645] core: Define interface of the tool for working with netmap keys

Announced keys of storage nodes are required for many application components
to function.

Define a unified interface for the utility for working with public keys of
nodes. Add a method to check if the key has been advertised by the local
node in the application context.

Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>

pkg/core/netmap/keys.go

@@ -0,0 +1,7 @@
+package netmap
+
+// AnnouncedKeys is an interface of utility for working with announced public keys of the storage nodes.
+type AnnouncedKeys interface {
+	// Checks if key was announced by local node.
+	IsLocalKey(key []byte) bool
+}